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Re^2: Is map's sub block really being evaluated in list context? wantarray() returns undef..

by moritz (Cardinal)
on Oct 20, 2011 at 10:35 UTC ( #932615=note: print w/replies, xml ) Need Help??


in reply to Re: Is map's sub block really being evaluated in list context? wantarray() returns undef..
in thread Is map's sub block really being evaluated in list context? wantarray() returns undef..

Curiously, if you define your own sub which takes a block, wantarray treats it as a normal routine:
$ perl -wE 'sub f(&) { shift->() }; say f { wantarray }' 1

I wonder if this inconsistency is worth a bug report.

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Re^3: Is map's sub block really being evaluated in list context? wantarray() returns undef..
by BrowserUk (Pope) on Oct 20, 2011 at 15:07 UTC

    I suspect that as a built-in, map avails itself of more optimisations than it would be safe to apply to a user-written function.

    I know from experience that if you implement a callback interface in XS using the full belts'n'braces approach described in perlcall, then the result is far less efficient than the carefully optimised callback interface used by List::Util::reduce().

    The downside of trying to match that level of optimisation with your own XS code is that it is very easy to create the situation whereby the callback leaks memory if it contains closures. (The early versions of reduce() suffered from the same problems.)

    I think it would be very difficult, if not impossible to compile user-written, runtime-compiled subs taking callbacks, to be as efficient as map. And to 'fix the inconsistency' would essentially require pessimising the implementation of map

    So no, I don't think it is worth a bug report. At most, a documentation change noting the difference might be useful.


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